I don't have an air horn yet, but I am totally not joking about wanting one. No, it wouldn't be the first warning. They still get the bell (if I'm on my commuter), and when they ignore the bell, they get my voice, and when they ignore my voice, THEN they get the air horn.

I want one for running, too. Both for the oblivious people taking up the entire MUP or track, and for the target-fixated drivers on the road.